1. Field of the Invention
The present invention relates to a driving torque transmitting device and more particularly to a driving torque transmitting device suitably employed in a four wheel drive car.
2. Description of the prior art
Four wheel drive(4WD)cars can run through snowy roads and rough roads with excellent stability by distributing driving torque to front and rear four wheels. These 4WD cars have been used as so-called off road cars. And while they are running on well-conditioned roads, 4WD has been changed to 2WD to prevent tight corner braking phenomenon or the like by manually operating a select lever.
Recently, the running performance of normal road cars has been demanded to be improved and 4WD has been employed in these cars for complying with the above demand. Under the above circumstances, there has been dynamically developed such a driving torque distributing device as to substantially disconnect front and rear wheels or right and left wheels from each other while running on well-conditioned roads without troublesome manual operation of the select lever.
As one example of the above described driving torque distributing device, there has been proposed the device provided with a viscous coupling by which axle shafts are brought into a nearly directly coupled condition only when a large difference in the number of rotation is generated between the axle shafts. This device is comparatively simple in structure, but it is difficult to obtain remarkable effect of 4WD since the axle shafts are not coupled to each o&her mechanically.
U.S. Pat. No. 2,949,792 discloses a differential assembly exhibiting remarkable 4WD effect, wherein pump assemblies, each being composed of a plurality of gears, pistons, clutch plates to be compressed by the pistons are provided within a differential case. When a large difference in the number of rotation is generated between axle shafts, the pump assemblies are operated to supply high pressure oil to the pistons. Then, the clutch plates are compressed to mechanically couple the axle shafts to the differential case.
However, the structure of the differential assembly of U.S. Pat. No. 2,949,792 is very complex and it is required to form a plurality of passages in the differential case for conveying pumped fluid from the pump assemblies to the distant pistons. Accordingly, this differential assembly has problems that the overall device becomes large and complex and that the production cost thereof is expensive.